The present invention relates to a method of forming a silicon oxide layer in the production of, for example, a semiconductor device.
In the production of, for example, a metal oxide semiconductor (MOS) device, it is required to form a gate oxide of silicon oxide on a surface of a silicon semiconductor substrate. Further, in the production of a thin film transistor (TFT), it is required to form a gate oxide of silicon oxide on a surface of a silicon layer provided on an insulating substrate as well. It can be said that a silicon oxide layer fully takes part in the reliability of a semiconductor device. A silicon oxide layer is therefore constantly required to have high dielectric breakdown resistance and long-term reliability.
For example, when a MOS semiconductor device is produced, conventionally, a surface of a silicon semiconductor substrate is cleaned by RCA cleaning prior to the formation of a gate oxide. In the RCA cleaning, the surface of the silicon semiconductor substrate is cleaned with an NH4OH/H2O2 aqueous solution and then further cleaned with an HCl/H2O2 aqueous solution to remove fine particles and metal impurities from the surface. Meanwhile, when the RCA cleaning is carried out, the surface of the silicon semiconductor substrate reacts with the cleaning liquid to form a silicon oxide layer having a thickness of approximately 0.5 nm to 1 nm. This silicon oxide layer will be simply referred to as xe2x80x9coxide layerxe2x80x9d hereinafter. The oxide layer is non-uniform in thickness and contains a residual component of the cleaning liquid. The oxide layer is therefore removed by immersing the silicon semiconductor substrate in a hydrofluoric acid aqueous solution, and further, a chemical component is removed with pure water. As a result, there can be obtained a silicon semiconductor substrate surface which is mostly terminated with hydrogen and only partly terminated with fluorine. In the present specification, obtaining a silicon semiconductor substrate surface which is mostly terminated with hydrogen and only partly terminated with fluorine will be represented as xe2x80x9cexposing the surface of a silicon semiconductor substratexe2x80x9d. Thereafter, the above-obtained silicon semiconductor substrate is introduced into a process chamber (oxidation chamber) to form a silicon oxide layer on its surface.
With a decrease in thickness of a gate oxide and an increase in diameter of a substrate, an apparatus for the formation of a silicon oxide layer has been being converted from a horizontal-type apparatus in which a process chamber extends in the horizontal direction to a vertical-type apparatus in which a process chamber extends in the vertical direction. The horizontal-type apparatus for the formation of a silicon oxide layer is referred to as xe2x80x9chorizontal-type processing apparatusxe2x80x9d hereinafter, and the vertical-type apparatus for the formation of a silicon oxide layer is referred to as xe2x80x9cvertical-type processing apparatusxe2x80x9d hereinafter. The reason therefor is as follows. Not only the vertical-type processing apparatus can easily cope with an increase in the diameter of a substrate as compared with the horizontal-type processing apparatus, but also the vertical-type processing apparatus can serve to decrease the formation of a layer of silicon oxide caused by atmosphere taken into a process chamber of the vertical-type processing apparatus during the transfer of a silicon semiconductor substrate into the process chamber. The above layer of silicon oxide will be referred to as xe2x80x9cnatural oxidexe2x80x9d hereinafter. However, even the use of the vertical-type processing apparatus results in the formation of a natural oxide having a thickness of approximately 2 nm on the surface of the silicon semiconductor substrate. The natural oxide contains a large amount of impurities derived from atmosphere, and the presence of the natural oxide is not at all negligible when a gate oxide is decreased in thickness. There have been therefore proposed methods for preventing the formation of the natural oxide to the lowest level possible, such as (1) a method in which a nitrogen gas atmosphere is formed in a substrate transfer portion provided in a vertical-type processing apparatus by flowing a large volume of nitrogen gas (nitrogen gas purge method), and (2) a method in which a substrate transfer portion is vacuumed and then inert gas such as nitrogen gas is introduced into the substrate transfer portion (vacuum loadlock method).
Thereafter, in a state where an inert gas atmosphere is formed in the process chamber (oxidation chamber), a silicon semiconductor substrate is brought into the process chamber. Then, an atmosphere of the process chamber is replaced with an oxidative atmosphere to form a gate oxide. For the formation of the gate oxide, there is generally employed a method in which a surface of a silicon semiconductor substrate is thermally oxidized by introducing high-purity water vapor into the process chamber maintained at a high temperature (wet oxidation method). In this method, a gate oxide having high electric reliability can be obtained as compared with a method in which the surface of a silicon semiconductor substrate is oxidized with dry oxygen gas (dry oxidation method). Included in the above wet oxidation method is an oxidation method using pyrogenic gas (hydrogen gas combustion oxidation method) in which hydrogen gas is mixed with oxygen gas at a high temperature and is combusted and the so-generated water vapor is used for oxidizing silicon in the silicon semiconductor substrate. The oxidation method using pyrogenic gas is widely used. In the oxidation method using pyrogenic gas, generally, oxygen gas is supplied into a combustion chamber disposed outside the process chamber and being maintained at 700 to 900xc2x0 C., and then hydrogen gas is supplied into the combustion chamber to combust the hydrogen gas with the oxygen gas at a high temperature. The so-obtained water vapor is used as oxidizing species.
FIG. 13 shows a schematic view of a vertical-type processing apparatus based on an oxidation method using pyrogenic gas. The vertical-type processing apparatus comprises a double-tubular structured process chamber 10 which is made of fused quartz and perpendicularly held, a gas inlet port 12 for introducing water vapor and/or gas into the process chamber 10, a gas exhaust port 13 for exhausting the water vapor and/or the gas from the process chamber 10, a heater 14 for maintaining the interior of the process chamber 10 at a predetermined ambient temperature through a cylindrical liner tube 16 made of SiC, a substrate transfer portion 20, a gas introducing portion 21 for introducing inert gas such as nitrogen gas into the substrate transfer portion 20, a gas exhaust portion 22 for exhausting the gas from the substrate transfer portion 20, a shutter 15 for partitioning the process chamber 10 and the substrate transfer portion 20, and an elevator unit 23 for bringing silicon semiconductor substrates into and out of the process chamber 10. Attached to the elevator unit 23 is a fused quartz boat 24 on which the silicon semiconductor substrates are to be placed. Further, hydrogen gas supplied to a combustion chamber 30 is mixed with oxygen gas at a high temperature and combusted in the combustion chamber 30, to generate water vapor. The water vapor is introduced into the process chamber 10 through a piping 31, a gas passage 11 and the gas inlet port 12. The gas passage 11 corresponds to a space between an inner wall and an outer wall of the double-tubular structured process chamber 10.
The outline of a conventional method of forming a silicon oxide layer with the vertical-type processing apparatus based on an oxidation method using pyrogenic gas will be explained below with reference to FIG. 13 and FIGS. 82, 83 and 84.
[Step-10]
Nitrogen gas is introduced into the process chamber 10 through a piping 32, the combustion chamber 30, the piping 31, the gas passage 11 and the gas inlet port 12 so as to bring a nitrogen atmosphere into the process chamber 10, and the ambient temperature in the process chamber 10 is maintained at 700 to 800xc2x0 C. with the heater 14 through the liner tube 16. In this state, the shutter 15 is kept closed (see FIG. 82A). The substrate transfer portion 20 is in a state where it is open to atmosphere.
[Step-20]
Silicon semiconductor substrates 40 are transferred into the substrate transfer portion 20, and placed on the fused quartz boat 24. After the transfer of the silicon semiconductor substrates 40 into the substrate transfer portion 20 is completed, a door (not shown) is closed. Then, nitrogen gas is introduced into the substrate transfer portion 20 through the gas introducing portion 21 and is exhausted through the gas exhaust portion 22, so that the atmosphere of the substrate transfer portion 20 is replaced with a nitrogen gas atmosphere (see FIG. 82B).
[Step-30]
When a sufficient nitrogen gas atmosphere is formed inside the substrate transfer portion 20, the shutter 15 is opened (see FIG. 83B), and the elevator unit 23 is actuated to elevate the fused quartz boat 24, whereby the silicon semiconductor substrates 40 are transferred into the process chamber 10 (see FIG. 84A). When the elevator unit 23 reaches its uppermost position, the base portion of the fused quartz boat 24 prevents the communication between the process chamber 10 and the substrate transfer portion 20.
If the interior of the process chamber 10 is left in a nitrogen gas atmosphere when the shutter 15 is opened, the following problem occurs. That is, when a silicon semiconductor substrate of which a surface is exposed with a hydrofluoric acid aqueous solution and pure water is introduced into a nitrogen gas atmosphere having a high temperature, the silicon semiconductor substrate 40 undergoes surface roughening. This phenomenon is assumed to be caused for the following reason. Part of Sixe2x80x94H bonds and Sixe2x80x94F bonds, which are formed on the surface of the silicon semiconductor substrate 40 by the cleaning with a hydrofluoric acid aqueous solution and pure water, are eliminated due to the elimination of hydrogen and/or fluorine caused by an increase in temperature, to cause an etching phenomenon on the surface of the silicon semiconductor substrate 40. For example, xe2x80x9cUltraclean ULSI Technologyxe2x80x9d (OMI Tadahiro, issued by Baifukan), page 21, describes that, when a silicon semiconductor substrate is temperature-increased to 600xc2x0 C. or higher in an argon gas atmosphere, a heavy concave or convex shape is formed on a surface of the silicon semiconductor substrate. For preventing the above phenomenon, for example, nitrogen gas containing 0.5 vol % of oxygen gas is introduced into the process chamber 10 through the gas inlet port 12 before the shutter 15 is opened, thereby to form a nitrogen gas atmosphere containing 0.5 vol % of oxygen gas inside the process chamber 10 (see FIG. 83A).
[Step-40]
Then, the ambient temperature inside the process chamber 10 is raised to 800 to 900xc2x0 C. Oxygen gas and hydrogen gas are supplied to the combustion chamber 30 through the pipings 32 and 33, and the hydrogen gas is mixed with the oxygen gas at a high temperature and combusted in the combustion chamber 30. The so-generated water vapor is introduced into the process chamber 10 through the piping 31, the gas passage 11 and the gas inlet port 12, and is exhausted through the gas exhaust port 13 (see FIG. 84B), whereby a silicon oxide layer is formed on the surface of each silicon semiconductor substrate 40. For preventing the occurrence of a detonating gas reaction of incomplete-combusted hydrogen gas in the process chamber 10, which incomplete-combusted hydrogen gas may flow into the process chamber 10 prior to the introduction of water vapor into the process chamber 10, oxygen gas is introduced into the combustion chamber 30 through the piping 32 before hydrogen gas is introduced into the combustion chamber 30 through the piping 33. As a result, the oxygen gas flows into the process chamber 10 through the piping 31, the gas passage 11 and the gas inlet port 12. The temperature in the combustion chamber 30 is maintained at 700 to 900xc2x0 C., for example, with a heater (not shown).
Since a nitrogen gas atmosphere containing about 0.5 vol % of oxygen gas is formed in the process chamber 10 by introducing nitrogen gas containing about 0.5 vol % of oxygen gas into the process chamber 10 through the gas inlet port 12 before the shutter 15 is opened, the formation of a concave or convex shape on the surface of each silicon semiconductor substrate can be prevented. According to xe2x80x9cUltraclean ULSI Technologyxe2x80x9d (OMI Tadahiro, issued by Baifukan), page 21, the formation of a concave or convex shape on a surface of a silicon semiconductor substrate can be also prevented by dry-oxidizing the silicon semiconductor substrate having a surface terminated with hydrogen, at 300xc2x0 C. at which the terminal hydrogen is stably present and thereby forming a silicon oxide layer as a protective layer.
Since, however, nitrogen gas which contains oxygen gas is introduced into the process chamber 10 for preventing the phenomenon of formation of a concave or convex shape on the surface of each silicon semiconductor substrate, a silicon oxide layer is formed on the surface of the silicon semiconductor substrate transferred into the process chamber 10. The so-formed silicon oxide layer is essentially a silicon oxide layer formed by dry oxidation (called xe2x80x9cdry oxide layerxe2x80x9d), and its properties are inferior to those of a silicon oxide layer formed by a wet oxidation method (called xe2x80x9cwet oxide layerxe2x80x9d). For example, when a silicon semiconductor substrate is transferred into the process chamber 10 while the ambient temperature of the process chamber 10 is kept at a temperature of 800xc2x0 C. and the atmosphere of the process chamber 10 is a nitrogen gas atmosphere containing 0.5 vol % of oxygen gas, a dry oxide layer having a thickness of 2 nm or more is formed on the surface of the silicon semiconductor substrate. It is expected that a semiconductor device having a gate length of 0.18 xcexcm to 0.13 xcexcm uses a gate oxide of 4 nm-3 nm thickness. When a gate oxide having a thickness of 4 nm is formed, 50% or more of the thickness of the gate oxide is formed of the dry oxide layer.
JP-A-6-291112 discloses a method for overcoming the above problem. That is, in this method, a silicon semiconductor substrate is cleaned with a hydrofluoric acid aqueous solution and then immersed in a hydrogen peroxide aqueous solution to form a silicon oxide layer as a protective layer on a surface of the silicon semiconductor substrate. In this method, however, it is difficult to form a uniform silicon oxide layer with good reproducibility by controlling the concentration of the hydrogen peroxide aqueous solution and some other means. Further, there is another problem that impurities contained in the hydrogen peroxide aqueous solution are included in the silicon oxide layer.
For example, JP-A-6-318588 discloses a method of forming a silicon oxide layer having excellent stability for a long period of time, having high dielectric breakdown resistance and having a small thickness. In this method, an ultra-thin thermal oxidation silicon layer is formed on a surface of a silicon semiconductor substrate by a thermal oxidation method, then, a silicon oxide layer is deposited on the thermal oxidation silicon layer by a chemical vapor deposition (CVD) method. Then, the deposited silicon oxide layer is heat-treated in an oxidative atmosphere. The above method has a problem that the process of forming a silicon oxide layer is complicated since the silicon oxide layer is deposited by the chemical vapor deposition (CVD) method.
The above problems occur not only on a surface of a silicon semiconductor substrate but also on a surface of a silicon layer formed on an insulating substrate or an insulating layer.
It is therefore an object of the present invention to provide a method of forming a silicon oxide layer having excellent properties on a surface of a silicon layer, in which the occurrence of surface roughening (a concave or convex shape) can be prevented and the formation of a dry oxide on the surface of the silicon layer can be prevented.
A method of forming a silicon oxide layer according to the first aspect of the present invention for achieving the above object comprises initiating formation of a silicon oxide layer on a surface of a silicon layer by an oxidation method using wet gas at an ambient temperature at which no silicon atom is eliminated from the surface of the silicon layer, and then forming the silicon oxide layer up to a predetermined thickness by an oxidation method using wet gas.
In a method according to the first aspect of the present invention, the ambient temperature at which no silicon atom is eliminated from the surface of the silicon layer is preferably a temperature at which bond of an atom terminating the surface of the silicon layer and a silicon atom is not broken. Specifically, the temperature at which no silicon atom is eliminated from the surface of the silicon layer is preferably a temperature at which Sixe2x80x94H bond on the surface of the silicon layer is not broken or a temperature at which Sixe2x80x94F bond on the surface of the silicon layer is not broken. When a silicon semiconductor substrate having a (100) crystal orientation is used, such a silicon semiconductor substrate has a termination structure in which most of hydrogen atoms on the surface of the silicon semiconductor substrate bond to silicon atoms in a manner that two hydrogen atoms bond to one silicon atom such as Hxe2x80x94Sixe2x80x94H. In a portion, for example, a STEP-formed portion where the surface state of the silicon semiconductor substrate is collapsed, however, there is formed a termination structure in which only one bond of a silicon atom is bound to a hydrogen atom or a termination structure in which each of three bonds of a silicon atom is bound to a hydrogen atom. Generally, remaining bond or bonds of each silicon atom on the surface of the silicon semiconductor substrate is or are bound to a silicon atom or silicon atoms inside a crystal. The term xe2x80x9cSixe2x80x94H bondxe2x80x9d in the present specification includes all of a termination structure in which each of two bonds of a silicon atom is bound to a hydrogen atom, a termination structure in which only one bond of a silicon atom is bound to a hydrogen atom and a termination structure in which each of three bonds of a silicon atom is bound to a hydrogen atom. In view of a throughput, more specifically, the ambient temperature at which the formation of a silicon oxide layer on the surface of the silicon layer is initiated is a temperature at which the wet gas does not undergo condensation on the surface of the silicon layer, preferably at least 200xc2x0 C., more preferably at least 300xc2x0 C.
A method of forming a silicon oxide layer according to the second aspect of the present invention for achieving the above object comprises initiating formation of a silicon oxide layer on the surface of a silicon layer by an oxidation method using wet gas at an ambient temperature which is equivalent to, or higher than, a temperature at which the wet gas does not undergo condensation on the surface of the silicon layer and is equivalent to, or lower than, 500xc2x0 C., preferably 450xc2x0 C., more preferably 400xc2x0 C., and then forming the silicon oxide layer up to a predetermined thickness by an oxidation method using wet gas.
In a method according to the first or second aspect of the present invention, the oxidation method using wet gas is at least one selected from an oxidation method using pyrogenic gas, an oxidation method using water vapor generated by heating pure water and an oxidation method using water vapor generated by bubbling hot pure water with oxygen gas or inert gas. Since the oxidation method using wet gas is employed, there can be obtained a silicon oxide layer having an excellent time dependent dielectric breakdown (TDDB) property. In the oxidation method using wet gas, the wet gas may be diluted with inert gas such as nitrogen gas, argon gas or helium gas.
In a method according to the first or second aspect of the present invention, the ambient temperature when the formation of the silicon oxide layer having the predetermined thickness is completed is preferably higher than the ambient temperature at which the formation of a silicon oxide layer is initiated. Although not specially limited, the ambient temperature when the formation of the silicon oxide layer having the predetermined thickness is completed is 600 to 1200xc2x0 C., preferably 700 to 1000xc2x0 C., more preferably 750 to 900xc2x0 C.
In a method according to the first or second aspect of the present invention, after the formation of the silicon oxide layer having the predetermined thickness is completed, heat treatment of the silicon oxide layer is preferably carried out for further improving its properties.
An atmosphere for the heat treatment in the above case is preferably an inert gas atmosphere containing a halogen element. When the silicon oxide layer is heat-treated in the inert gas atmosphere containing a halogen element, the resultant silicon oxide layer has an excellent time-zero dielectric breakdown (TZDB) property and an excellent time dependent dielectric breakdown (TDDB) property. The inert gas used for the above heat treatment includes nitrogen gas, argon gas and helium gas. The halogen element includes chlorine, bromine and fluorine, and chlorine is preferred. The form of the halogen element contained in the inert gas may include hydrogen chloride (HCl), CCl4, C2HCl3, Cl2, HBr and NF3. The content of the halogen element in the inert gas in terms of molecules or compounds is 0.001 to 10 vol %, preferably 0.005 to 10 vol %, more preferably 0.02 to 10 vol %. When, for example, chlorine is introduced as hydrogen chloride gas, the content of the hydrogen chloride gas in the inert gas is preferably 0.02 to 10 vol %.
In a method according to the first or second aspect of the present invention, the heat treatment may be carried out with a so-called single wafer processing, while the heat treatment with furnace annealing is preferred. The ambient temperature for the heat treatment is 700 to 1200xc2x0 C., preferably 700 to 1000xc2x0 C., more preferably 700 to 950xc2x0 C. When the heat treatment is carried out with furnace annealing, the heat treatment time is 5 to 60 minutes, preferably 10 to 40 minutes, more preferably 20 to 30 minutes. When the heat treatment is carried out with a single wafer processing, the heat treatment time is preferably 1 to 10 minutes.
In a method according to the first or second aspect of the present invention, the ambient temperature for the heat treatment of the formed silicon oxide layer is preferably higher than the ambient temperature at which the formation of the silicon oxide layer having the predetermined thickness is completed. In this case, after the formation of the silicon oxide layer having the predetermined thickness is completed, the atmosphere is replaced with an inert gas atmosphere, and then, the ambient temperature may be raised to a temperature for the heat treatment. Further, it is preferred to raise the ambient temperature to a temperature for the heat treatment after the atmosphere is replaced with an inert gas atmosphere containing a halogen element. The inert gas includes nitrogen gas, argon gas and helium gas. The form of the halogen element contained in the inert gas may include hydrogen chloride (HCl), CCl4, C2HCl3, Cl2, HBr and NF3. The content of the halogen element in the inert gas in terms of molecules or compounds is 0.001 to 10 vol %, preferably 0.005 to 10 vol %, more preferably 0.02 to 10 vol %. When chlorine is introduced as hydrogen chloride gas for example, the content of the hydrogen chloride gas in the inert gas is preferably 0.02 to 10 vol %.
The heat treatment may be carried out in an inert gas atmosphere containing a halogen element under reduced pressure in which the pressure of the inert gas containing a halogen element is lower than atmospheric pressure.
After the heat treatment, the silicon oxide layer may be subjected to nitridation. The nitridation is preferably carried out in an N2O gas, NO gas or NO2 gas atmosphere, and it is most preferably carried out in an N2O gas atmosphere. Alternatively, it is preferred to carry out the nitridation in an NH3 gas, N2H4 or hydrazine derivative atmosphere and then to carry out an annealing in an N2O gas or O2 atmosphere. Desirably, the nitridation is carried out at a temperature of 700 to 1200xc2x0 C., preferably 800 to 1150xc2x0 C., more preferably 900 to 1100xc2x0 C. In this case, the silicon layer is preferably heated by irradiation with infrared ray or by treatment with furnace annealing.
Further, alternately, an atmosphere for the heat treatment may be a nitrogen-containing gas atmosphere. The nitrogen-containing gas includes N2, NH3, N2O, NO2 and NO.
A preferred embodiment of the method according to the first aspect of the present invention may include a first step of forming a silicon oxide layer and a second step of forming a silicon oxide layer, in which
the first step of forming a silicon oxide layer comprises initiating formation of a silicon oxide layer on the surface of the silicon layer by an oxidation method using wet gas at an ambient temperature at which no silicon atom is eliminated from the surface of the silicon layer, and then maintaining the atmosphere at an ambient temperature range in which no silicon atom is eliminated from the surface of the silicon layer, for a predetermined period of time, to form the silicon oxide layer, and
the second step of forming a silicon oxide layer comprises further forming the silicon oxide layer up to the predetermined thickness by an oxidation method using wet gas at an ambient temperature higher than the ambient temperature range in which no silicon atom is eliminated from the surface of the silicon layer. The first step of forming a silicon oxide layer is simply referred to as xe2x80x9cfirst oxidation stepxe2x80x9d hereinafter, and the second step of forming a silicon oxide layer is simply referred to as xe2x80x9csecond oxidation stepxe2x80x9d hereinafter.
In the above preferred embodiment of the present invention, the ambient temperature at which no silicon atom is eliminated from the surface of the silicon layer is preferably a temperature at which bond of an atom terminating the surface of the silicon layer and a silicon atom is not broken. Specifically, the temperature at which no silicon atom is eliminated from the surface of the silicon layer is preferably a temperature at which Sixe2x80x94H bond on the surface of the silicon layer is not broken or a temperature at which Sixe2x80x94F bond on the surface of the silicon layer is not broken. In view of a throughput, more specifically, the ambient temperature at which the formation of a silicon oxide layer on the surface of the silicon layer is initiated is a temperature at which the wet gas does not undergo condensation on the surface of the silicon layer, preferably at least 200xc2x0 C., more preferably at least 300xc2x0 C. Further, in the first oxidation step, in the second oxidation step or in the first and second oxidation steps, the oxidation method using wet gas is preferably at least one selected from an oxidation method using pyrogenic gas, an oxidation method using water vapor generated by heating pure water and an oxidation method using water vapor generated by bubbling hot pure water with oxygen gas or inert gas. Since the oxidation method using wet gas is employed, there can be obtained a silicon oxide layer having an excellent time dependent dielectric breakdown (TDDB) property. The first oxidation step and the second oxidation step may use the same oxidation method or different oxidation methods. The wet gas used in the first oxidation step, in the second oxidation step or in the first and second oxidation steps may be diluted with inert gas such as nitrogen gas, argon gas or helium gas.
The wet gas used in the first oxidation step, in the second oxidation step or in the first and second oxidation steps may contain a halogen element. When the wet gas containing a halogen element is used, there can be obtained a silicon oxide layer having an excellent time-zero dielectric breakdown (TZDB) property and an excellent time dependent dielectric breakdown (TDDB) property. The halogen element includes chlorine, bromine and fluorine, and chlorine is preferred. The form of the halogen element contained in the wet gas may include hydrogen chloride (HCl), CCl4, C2HCl3, Cl2, HBr and NF3. The content of the halogen element in the inert gas in terms of molecules or compounds is 0.001 to 10 vol %, preferably 0.005 to 10 vol %, more preferably 0.02 to 10 vol %. When chlorine is introduced as hydrogen chloride gas for example, the content of the hydrogen chloride gas in the wet gas is preferably 0.02 to 10 vol %.
In the preferred embodiment of the present invention, the temperature for forming a silicon oxide layer in the second oxidation step is 600 to 1200xc2x0 C., preferably 700 to 1000xc2x0 C., more preferably 750 to 900xc2x0 C.
In the preferred embodiment of the present invention, an apparatus for the formation of a silicon oxide layer having one process chamber is used, and the first step and second oxidation steps can be carried out in the process chamber. The above embodiment will be referred to as a preferred first embodiment of the present invention. In this case, it is preferred to carry out the first and second oxidation steps with a batch processing. This embodiment will be referred to as a preferred first embodiment A of the present invention.
In the preferred first embodiment of the present invention, the process chamber is provided with heating means for heating the silicon layer, the heating means is disposed outside the process chamber and nearly in parallel with the surface of the silicon layer, and the first and second oxidation steps can be carried out with a single wafer processing. This embodiment will be referred to as a preferred first embodiment B of the present invention.
The preferred first embodiment of the present invention preferably includes a heating-up step between the first oxidation step and the second oxidation step. In this case, an atmosphere in the heating-up step is preferably an inert gas atmosphere or an atmosphere under reduced pressure, or it is preferably an oxidative atmosphere containing wet gas. The inert gas includes nitrogen gas, argon gas and helium gas. The inert gas or the wet gas in the atmosphere in the heating-up step may contain a halogen element. Owing to the presence of a halogen element, the silicon oxide layer formed in the first oxidation step can be further improved in properties. That is, a silicon dangling bond (Si.) and Sixe2x80x94OH which are defects caused in the first oxidation step react with a halogen element at the heating-up step, so that the silicon dangling bond is terminated or a dehydrating reaction is caused. As a result, these defects as reliability-decreasing factors are removed. In particular, the removal of the above defects is effectively applied to an initial silicon oxide layer formed in the first oxidation step. The halogen element includes chlorine, bromine and fluorine, and chlorine is preferred. The form of the halogen element contained in the inert gas or the wet gas may include hydrogen chloride (HCl), CCl4, C2HCl3, Cl2, HBr and NF3. The content of the halogen element in the inert gas or the wet gas in terms of molecules or compounds is 0.001 to 10 vol %, preferably 0.005 to 10 vol %, more preferably 0.02 to 10 vol %. When chlorine is introduced as hydrogen chloride gas for example, the content of the hydrogen chloride gas in the inert gas or the wet gas is preferably 0.02 to 10 vol %. The atmosphere in the heating-up step may be an oxidative atmosphere containing the wet gas diluted with inert gas.
Alternatively, in the preferred embodiment of the present invention, used is an apparatus for the formation of a silicon oxide layer which has a first process chamber for forming a silicon oxide layer, a second process chamber for forming a silicon oxide layer and a transfer passage connecting the first process chamber and the second process chamber. And, the first oxidation step is carried out in the first process chamber, the silicon layer is transferred from the first process chamber to the second process chamber through the transfer passage, and then, the second oxidation step is carried out in the second process chamber. The above embodiment will be referred to as a preferred second embodiment of the present invention.
In the preferred second embodiment of the present invention, it is preferred to transfer the silicon layer from the first process chamber to the second process chamber through the transfer passage without exposing the silicon layer to atmosphere in view of the prevention of contamination of the surface of the formed silicon oxide layer. An atmosphere inside the transfer passage during the transfer of the silicon layer is preferably an inert gas atmosphere or an atmosphere under reduced pressure. The inert gas includes nitrogen gas, argon gas and helium gas. In this case, the ambient temperature in the transfer passage during the transfer of the silicon layer from the first process chamber to the second process chamber may be room temperature (ordinary temperature), while it is preferably a temperature nearly equivalent to the ambient temperature at which a silicon oxide layer is formed on the surface of the silicon layer in the first process chamber, in view of an improvement in a throughput.
In the preferred second embodiment of the present invention, the first and second oxidation steps may be carried out with a batch processing. Otherwise, the first oxidation step may be carried out with a single wafer processing, and the second oxidation step may be carried out with a batch processing. Further, the first and second oxidation steps may be carried out with a single wafer processing.
The preferred second embodiment of the present invention may use the apparatus for the formation of a silicon oxide layer, in which a shutter is further disposed between that portion of the transfer passage which communicates with the first process chamber and that portion of the transfer passage which communicates with the second process chamber.
In the preferred embodiment of the present invention, it is preferred to heat-treat the silicon oxide layer after the second oxidation step is completed. The heat treatment can be carried out in the same manner as in the first or second aspect of the present invention, and its detailed explanation is therefore omitted. The second oxidation step and the heat treatment may be carried out in the same process chamber or may be carried out in different process chambers. Although not specially limited, examples of combinations of preferred processings in the first and second oxidation steps and in the heat treatment are as shown in Table 1 below. In Table 1 and Table 5, xe2x80x9cBatchxe2x80x9d means a batch processing and xe2x80x9cSinglexe2x80x9d means a single wafer processing.
In the preferred embodiment of the present invention, the final thickness of the silicon oxide layer obtained after the second oxidation step can be a thickness which a semiconductor device is required to have. On the other hand, the thickness of the silicon oxide layer obtained after the first oxidation step is preferably as thin as possible. However, a silicon semiconductor substrate used for the production of semiconductor devices has a (100) crystal orientation in most cases, and a (100) silicon semiconductor substrate necessarily has a so-called STEP on its surface whatever means is taken to smoothen and flatten the surface of the silicon semiconductor substrate. The above STEP is generally of a one silicon atom layer height, while a STEP of a few silicon atoms layer height is sometimes formed. Although not specially limited, it is therefore preferred to form a silicon oxide layer having a thickness of at least 1 nm in the first oxidation step when a (100) silicon semiconductor substrate is used as a silicon layer.
In a method according to the first aspect of the present invention including the preferred embodiment of the present invention, or in a method according to the second aspect of the present invention, an atmosphere before the formation of a silicon oxide is preferably an atmosphere of inert gas such as nitrogen gas, argon gas or helium gas or an atmosphere under reduced pressure for preventing the formation of an undesirable silicon oxide layer before the formation of a silicon oxide layer based on the wet gas.
Generally, a surface of a silicon layer is cleaned by the RCA cleaning, which is composed of cleaning with an NH4OH/H2O2 aqueous solution and cleaning with HCl/H2O2, to remove fine particles and impurities of metals from the surface of the silicon layer before the formation of a silicon oxide layer on the surface of the silicon layer, and then the silicon layer is immersed in a hydrofluoric acid aqueous solution and then in pure water. However, when the so-prepared silicon layer is exposed to atmosphere, the surface of the silicon layer may be contaminated, water and organic substances may adhere to the surface of the silicon layer, or Si atoms on the surface of the silicon layer may bond to hydroxy groups (OH) (e.g., see xe2x80x9cHighly-reliable Gate Oxide Formation for Giga-Scale LSIs by using Closed Wet Cleaning System and Wet Oxidation with Ultra-Dry Unloadingxe2x80x9d, J. Yugami, et al., International Electron Device Meeting Technical Digest 95, pp. 855-858). When the formation of a silicon oxide layer is initiated in the above state, the formed silicon oxide layer includes water, organic substances or Sixe2x80x94OH bond, which may degrade the properties of the formed silicon oxide layer or cause a defective portion. The term xe2x80x9cdefective portionxe2x80x9d refers to a portion containing a silicon dangling bond (Si.) or Sixe2x80x94H bond in the silicon oxide layer or a portion containing Sixe2x80x94Oxe2x80x94Si bond which is compressed due to stress or has a bond angle different from that of Sixe2x80x94Oxe2x80x94Si in a thick or bulk silicon oxide layer. For preventing the above problems, preferably, the preferred embodiment of the present invention includes a step for cleaning the surface of the silicon layer before the first oxidation step, and the first oxidation step is initiated without exposing the surface-cleaned silicon layer to atmosphere (i.e., with keeping the silicon layer in an inert gas atmosphere or an atmosphere under reduced pressure from the cleaning of the surface of the silicon layer to the initiation of the first oxidation step). A method according to the first or second aspect of the present invention preferably includes a step for cleaning the surface of the silicon layer before forming a silicon oxide layer, and the formation of a silicon oxide layer is initiated without exposing the surface-cleaned silicon layer to atmosphere (i.e., with keeping the silicon layer in an inert gas atmosphere or an atmosphere under reduced pressure from the cleaning the surface of the silicon layer to the initiation of formation of a silicon oxide layer). As a result, a silicon oxide layer can be formed on the surface of the silicon layer terminated with hydrogen in most parts and terminated with fluorine in very small parts, so that the degradation of properties of the silicon oxide layer or the occurrence of the defective portion can be prevented.
FIGS. 1 to 11 schematically show ambient temperature profiles in the method of forming a silicon oxide layer in the present invention. In these Figures, the lower limit temperature of the ambient temperature at a time when the formation of a silicon oxide layer on the surface of the silicon layer is initiated is indicated as T1, and the upper limit temperature of the ambient temperature at which no silicon atom is eliminated from the surface of the silicon layer is indicated as T2. Further, the ambient temperature at which the formation of a silicon oxide layer having a predetermined thickness is completed or the ambient temperature in the second oxidation step is indicated as T3, and the ambient temperature in the heat treatment is indicated as T4. In these Figures, a solid line shows a state where a silicon oxide layer is being formed, and a chain line shows a process of increasing the ambient temperature up to the ambient temperature at which the formation of a silicon oxide layer on the surface of the silicon layer is initiated, a process of decreasing the ambient temperature to room temperature after the completion of the formation of a silicon oxide layer or a process of increasing the ambient temperature from that in the transfer step to the ambient temperature in the second oxidation step. A dotted line shows the transfer step in the transfer passage, and a doubled line shows the heat treatment step. An ambient temperature profile showing a heating-up step in a solid line shows that the formation of a silicon oxide layer is carried out in the heating-up step, and an ambient temperature profile showing a heating-up step in a short dashes line shows that the formation of a silicon oxide layer is not carried out in the heating-up step. In these Figures, further, xe2x80x9cRTxe2x80x9d stands for room temperature (ordinary temperature).
In examples of ambient temperature profiles shown in FIGS. 1A and 1B, the formation of a silicon oxide layer by an oxidation method using wet gas is initiated at an ambient temperature at which no silicon atom is eliminated from the surface of the silicon layer or at an ambient temperature at which the wet gas does not undergo condensation on the surface of the silicon layer and is equivalent to, or lower than, 500xc2x0 C. And, the silicon oxide layer is formed up to a predetermined thickness by the oxidation method using wet gas. The ambient temperature when the formation of the silicon oxide layer having a predetermined thickness is completed is the same as the ambient temperature at which the formation of a silicon oxide layer on the surface of the silicon layer was initiated (see FIG. 1A), or it is higher than the ambient temperature at which the formation of a silicon oxide layer on the surface of the silicon layer was initiated but within the temperature range in which no silicon atom is eliminated from the surface of the silicon layer (see FIG. 1B).
In examples of ambient temperature profiles shown in FIGS. 2A and 2B, the formation of a silicon oxide layer by an oxidation method using wet gas is initiated at an ambient temperature at which no silicon atom is eliminated from the surface of the silicon layer, or at an ambient temperature which is higher than a temperature at which the wet gas undergoes condensation on the surface of the silicon layer and is equivalent to, or lower than, 500xc2x0 C. And, the silicon oxide layer is formed up to a predetermined thickness by an oxidation method using wet gas. An ambient temperature (T3) when the formation of the silicon oxide layer having a predetermined thickness is completed is higher than the ambient temperature (T1-T2) at which the formation of the silicon oxide layer on the surface of the silicon layer was initiated, and it is higher than the ambient temperature range in which no silicon atom is eliminated from the surface of the silicon layer. In the example of the ambient temperature profile shown in FIG. 2A, the heating-up rate is constant. On the other hand, in the example of the ambient temperature profile shown in FIG. 2B, the heating-up rate is varied. The varying pattern of the heating-up rate is given for an illustration purpose, and shall not be limited to the pattern in FIG. 2B.
Examples of ambient temperature profiles shown in FIGS. 3 to 7 are concerned with the preferred first embodiment of the present invention, and a silicon oxide layer is formed in the first oxidation step, in the heating-up step in some cases and in the second oxidation step. In examples of FIGS. 3A and 3B, an ambient temperature in the first oxidation step and an ambient temperature in the second oxidation step are constant. In examples of FIGS. 4A and 4B, an ambient temperature in the first oxidation step is varied, while an ambient temperature in the second oxidation step is constant. In examples of FIGS. 5A and 5B, an ambient temperature in the first oxidation step is constant, while an ambient temperature in the second oxidation step is varied. In examples of FIGS. 6A and 6B, an ambient temperature in the first oxidation step and an ambient temperature in the second oxidation step are varied. The heating-up rate in the heating-up step may be constant or varied. Further, when the ambient temperature is varied in the first oxidation step and/or in the second oxidation step, the varying rate may be constant or varied. Alternatively, ambient temperature profiles shown in FIGS. 7A and 7B may be employed. In the example of the ambient temperature profile shown FIG. 7A, the heating-up rate is constant. On the other hand, in an example of the ambient temperature profile shown in FIG. 7B, the heating-up rate is varied. In addition, in the examples of FIGS. 3A, 4A, 5A and 6A, the heating-up step is carried out in an inert gas atmosphere, and a silicon oxide layer is not formed in the heating-up step. On the other hand, in examples of FIGS. 3B, 4B, 5B and 6B, the heating-up step is carried out in an oxidative atmosphere containing wet gas, and a silicon oxide layer is also formed in the heating-up step.
Examples of ambient temperature profiles shown in FIGS. 8 to 11 are concerned with the preferred second embodiment of the present invention. A silicon oxide layer is formed in the first oxidation step, and in the second oxidation. In the examples of FIGS. 8A and 8B, an ambient temperature in the first oxidation step and the ambient temperature in the second oxidation step are constant. In the examples of FIGS. 9A and 9B, an ambient temperature in the first oxidation step is varied, while an ambient temperature in the second oxidation step is constant. In the examples of FIGS. 10A and 10B, an ambient temperature in the first oxidation step is constant, while an ambient temperature in the second oxidation step is varied. In the examples of FIGS. 11A and 11B, an ambient temperature in the first oxidation step and an ambient temperature in the second oxidation step are varied. In the examples of FIGS. 8A, 9A, 10A and 11A, an ambient temperature in the transfer passage in the transfer step is, for example, room temperature (ordinary temperature). On the other hand, in the examples of FIGS. 8B, 9B, 10B and 11B, an ambient temperature in the transfer passage in the transfer step is nearly equivalent to the ambient temperature in the first process chamber in the first oxidation step.
In the method according to the first or second aspect of the present invention, the silicon layer includes not only substrates such as a silicon semiconductor substrate, but also silicon layers where a silicon oxide layer is to be formed, such as an epitaxial silicon layer including an epitaxial silicon layer formed by a selectively epitaxial growth, a polysilicon layer, an amorphous silicon layer, a silicon layer in a silicon-on-insulator (SOI) structure produced by a so-called wafer bonding method or by a xe2x80x9cSeparation by Implanted Oxygenxe2x80x9d (SIMOX) method, and a substrate or any layer of these where a semiconductor device or a semiconductor device component have been formed. The silicon semiconductor substrate may be produced by any one of a Czochralski (CZ) method, a magnetic field applied Czochralski (MCZ) method, a double layered Czochralski (DLCZ) method, a floating zone melting (FZ) method and the like. Further, a silicon semiconductor substrate of which the crystal defect is removed in advance by hydrogen annealing at a high temperature may be also used.
The method of forming a silicon oxide layer, provided by the present invention, can be applied to the formation of silicon oxide layers in various semiconductor devices, such as the formation of a gate oxide, a dielectric interlayer and a isolation region of a MOS transistor, the formation of a gate oxide of a top gate type or bottom gate type thin film transistor, and the formation of a tunnel gate oxide of a flash memory.
In the method of forming a silicon oxide layer according to the first aspect of the present invention, the formation of a silicon oxide layer on the surface of the silicon layer by an oxidation method using wet gas is initiated in an atmosphere which is maintained at a temperature at which no silicon atom is eliminated from the surface of the silicon layer. In the method of forming a silicon oxide layer according to the second aspect of the present invention, the formation of a silicon oxide layer on a surface of a silicon layer by an oxidation method using wet gas is initiated at an ambient temperature which is equivalent to, or higher than, a temperature at which the wet gas does not undergo condensation on the surface of the silicon layer and is 500xc2x0 C. or lower. Since the ambient temperature is set at the above temperature when the formation of a silicon oxide layer is initiated, the occurrence of a concave or convex shape on the surface of the silicon layer can be prevented. Further, the oxidation of silicon atoms begins not in the outermost surface of the silicon layer but with silicon atoms present one layer inside, i.e., with a so-called back bond. The flatness in the interface between the silicon layer and the silicon oxide layer is therefore maintained at an atomic level, and the finally formed silicon oxide layer has excellent properties. Further, since a silicon oxide layer is formed on the surface of the silicon layer by the oxidation method using wet gas, the finally formed silicon oxide layer contains no dry oxide layer, and has excellent properties.
In the method of forming a silicon oxide layer according to the first or second aspect of the present invention, it sometimes takes a long period of time to form a silicon oxide layer which fully satisfies the properties required of, for example, a gate oxide. In this case, the preferred embodiment of the present invention can be employed. In the preferred embodiment of the present invention, since the second oxidation step is carried out after the silicon oxide layer which works as a protective layer as well is formed on the surface of the silicon layer, a concave or convex shape (roughening) does not occur on the surface of the silicon layer, for example, in the heating-up step or in the transfer step in which the atmosphere is a non-oxidative atmosphere. Further, a silicon oxide layer having excellent properties, which can fully satisfy the properties required of a gate oxide, can be produced for a short period of time.
In the method according to the preferred first embodiment A of the present invention, there can be used a conventional vertical-type, resistance-heating apparatus for the formation of a silicon oxide layer, which apparatus has a process chamber (oxidation chamber) made of fused quartz and vertically held. When a vertical-type apparatus for the formation of a silicon oxide layer is used, for example, a heater is arranged outside a silicon semiconductor substrate in its circumferential direction and the peripheral portion of the silicon semiconductor substrate is more temperature-increased than the central portion thereof. As a result, when a silicon oxide layer is formed during the heating-up step, the peripheral portion of the silicon semiconductor substrate has a larger thickness than the central portion thereof. In the method according to the preferred first embodiment B of the present invention, a silicon layer is heated with heating means arranged nearly in parallel with the surface of the silicon layer, and the in-plane (within-a-wafer) variability of temperature on the silicon layer can be therefore decreased. As a result, even when a silicon oxide layer is formed during the heating-up step, the occurrence of the in-plane (within-a-wafer) variability in thickness of the silicon oxide layer can be prevented.
When the formation of a silicon oxide layer by two silicon oxidation steps is carried out in one process chamber, it is required to control the ambient temperature in the process chamber in a broad range, and it is sometimes difficult to accurately control the ambient temperature in the process chamber. Further, since it is required to raise the ambient temperature in the process chamber, the throughput is liable to decrease. In the preferred second embodiment of the present invention, a silicon oxide layer is formed in the first process chamber and the second process chamber. The ambient temperature in each process chamber can be therefore maintained at a constant temperature in a narrow range. Accordingly, not only the ambient temperature in each process chamber can be more accurately controlled, but also the temperature stability in each process chamber is excellent. Therefore, the preferred second embodiment of the present invention is excellent in the controllability of thickness of the silicon oxide layer. Moreover, unlike the formation of a silicon oxide layer in one process chamber by two silicon oxidation steps, it is not required to raise the temperature from the ambient temperature in the first oxidation step to the ambient temperature in the second oxidation step, and a decrease in the throughput is therefore not incurred.